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Old 09-29-2013, 12:54 PM   #1
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Right tape for the job?


The rigid foam insulation I'm gluing to my basement walls has a reflective side, which will face the inside. Should I use foil tape to seal the seams?

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Old 09-30-2013, 03:11 PM   #2
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Right tape for the job?


Foil side to concrete wall.

Foil faced foamboard; see #8; Rigid foam on basement walls - gap or no gap?

That tape will work. Better still; mesh tape/furnace mastic as the fb can shrink over time; http://www.buildingscience.com/docum...a-foam-shrinks Air seal the bottom/top of sheets; cover the concrete COMPLETELY. Not yelling, just showing the importance of it, lol.

Gary

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Old 10-01-2013, 02:13 PM   #3
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Right tape for the job?


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Originally Posted by Gary in WA View Post
Foil side to concrete wall.

Foil faced foamboard; see #8; Rigid foam on basement walls - gap or no gap?

That tape will work. Better still; mesh tape/furnace mastic as the fb can shrink over time; http://www.buildingscience.com/docum...a-foam-shrinks Air seal the bottom/top of sheets; cover the concrete COMPLETELY. Not yelling, just showing the importance of it, lol.

Gary
I'm considering returning the foil faced board for non-foil faced XPS, but I did want to ask you about this. The manufacturer site said to have the foil facing the air space, which I understand to be the 1-2" empty pocket between the insulation and the drywall (which would be facing inside):

http://building.dow.com/na/en/tools/...ns/masonry.htm

But you recommend the opposite. I'm curious as to why. Plus I'm confused where I've read they suggest it should face the "warm" side. I live in Ohio, so I want the inside warm in the winter and the inside cool in the summer.

Last edited by spaceman spif; 10-01-2013 at 02:16 PM.
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Old 10-01-2013, 08:42 PM   #4
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Right tape for the job?


The foil facing is a Class 1 vapor barrier. Vapor barriers need to face (be installed on) the moisture producing side- eg. heating climate= inside; cooling climate= outside. You don't need the reflective quality of the board facing for the added R-value (slight) but rather for its vapor barrier effectiveness quality. IF the other side is a vapor barrier also, either side would work. The concrete/f.b. interface is at 80-100%RH through most of the year, you want a f.b. that will slow the moisture yet allow it to pass through. Hence the link and pictures of PIC on the top 1/2 wall and XPS on bottom 1/2, to allow concrete drying somewhat without forcing saturation/capillarity/wicking of excess moisture to rot the wood mudsill plate because the concrete wall (think sponge) has reached full saturation of moisture. Fig. 11, 12; http://www.buildingscience.com/docum...lation-systems

The moisture drive is mostly inward with basements below grade; http://www.buildingscience.com/docum...ts?full_view=1 If the impermeable foil-facer was installed to the cavity, the board would absorb moisture being in direct contact with the concrete unless other side is also a vb, the board would wick water (PIC= 4-1/2 times more volume than XPS) and take a loss in retained R-value (PIC= 64% effective, XPS= 98%); http://msdssearch.dow.com/PublishedL...romPage=GetDoc
They are using the foil as a radiant barrier for minimal added R-value, hence the air space required (which can give convective looping or at least reduce cavity R-value due to air currents).

Gary
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Old 10-01-2013, 09:28 PM   #5
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Right tape for the job?


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Originally Posted by Gary in WA View Post
The foil facing is a Class 1 vapor barrier. Vapor barriers need to face (be installed on) the moisture producing side- eg. heating climate= inside; cooling climate= outside. You don't need the reflective quality of the board facing for the added R-value (slight) but rather for its vapor barrier effectiveness quality. IF the other side is a vapor barrier also, either side would work. The concrete/f.b. interface is at 80-100%RH through most of the year, you want a f.b. that will slow the moisture yet allow it to pass through. Hence the link and pictures of PIC on the top 1/2 wall and XPS on bottom 1/2, to allow concrete drying somewhat without forcing saturation/capillarity/wicking of excess moisture to rot the wood mudsill plate because the concrete wall (think sponge) has reached full saturation of moisture. Fig. 11, 12; http://www.buildingscience.com/docum...lation-systems

The moisture drive is mostly inward with basements below grade; http://www.buildingscience.com/docum...ts?full_view=1 If the impermeable foil-facer was installed to the cavity, the board would absorb moisture being in direct contact with the concrete unless other side is also a vb, the board would wick water (PIC= 4-1/2 times more volume than XPS) and take a loss in retained R-value (PIC= 64% effective, XPS= 98%); http://msdssearch.dow.com/PublishedL...romPage=GetDoc
They are using the foil as a radiant barrier for minimal added R-value, hence the air space required (which can give convective looping or at least reduce cavity R-value due to air currents).

Gary
Excellent info! And now that I think about it, I remember reading this link before and it was seeing figure 13 that made me want to go with 2" XPS.

I can see from maps from other buildingscience.com links that I am located right on the border of the "cold" and the "mixed-humid" region. I can also tell I'm using the term XPS for the insulation I bought when it's actually a polyisocyanurate.

While I'm digging more into the info, I had another question pop into my head. A lot of what I've been reading seems to deal with concrete walls, but my basement walls are cinder block. Since cinder block has a hollow core, how does that affect the migration of water and water vapor?
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Old 10-01-2013, 10:29 PM   #6
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Right tape for the job?


If many of the CMU's are hollow, moisture will collect at the bottoms, sometimes requiring an inside drainage system- if they are filled with grout at install- they act similar to poured concrete, though not as dense--- easier capillarity IMO; http://www.buildingscience.com/docum...g-your-basment

#4, 5; http://www.extension.umn.edu/distrib...s/7051-04.html

If certain areas are wet at the wall/slab joint, figure an empty cell or two there- or a spring, change in soil porosity, lack of grout, etc.

Gary

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